Measurement of dimensions and properties of metal products is of vital importance in the metal industry of today. To be able to control the end products to the desired quality in the manufacturing processes, it is of great importance for the continuous measurement of certain quantities to be correct and reliable. This particularly applies to the manufacture of sheet or strip where, for example, the thickness is of vital significance. The technique that is used today is normally based on light or radiation or mechanical contact.
One such known method for non-contact measurement of the thickness of a sheet is to irradiate it with radioactive radiation or with X-ray radiation and then measure the radiation absorption of the sheet. This absorption is dependent on, among other things, the thickness of the sheet and hence constitutes a primary measured value of the thickness of the object to be measured. The measured value is, however, influenced by the material composition of the object to be measured, so the accuracy of measurement is not sufficiently good.
Known techniques are also sensitive to disturbances from the surrounding environment and are difficult to use when a high material quality is aimed at. A new fundamental measurement technology, which does not possess these deficiencies, is therefore desirable.
One such technique is inductive measurement technique. This has long been proposed as a possible measurement technique for measuring dimensions and properties of metals. The oldest patents in the field date back as early as 1920. However, this technique has met with limited success and it was not industrially accepted until the technique was further developed.
The measurement of, for example, thickness proved to be too dependent on material. With the technique disclosed, for example, in U.S. Pat. No. 5,059,902 and SE 517293, industrially successful measuring equipment could suddenly be designed, manufactured and used. These various types of measuring equipment have proved to work well and to be without the deficiencies from which the prior art measurement technique suffered.
However, also this new technique has proved to involve certain drawbacks. One disadvantage is, for example, that it has not been possible to use it for measurement on really thin sheets with thicknesses down to about 0.1 mm for copper and aluminium, that is, metal foil, and for somewhat larger thicknesses for metals with higher electrical resistivity. This is a significant drawback since an industrial measurement technique of this kind should be generally applicable and capable of being used for measuring on objects/sheets of all available thicknesses so as to avoid the need of installing and using several different types of measuring equipment.
With further developed technique, it has been found to be possible, using electromagnetic technique, to measure also really thin sheets. One problem when measuring on very thin sheets, such as metal foils, is that the time of penetration of the magnetic fields, that is, the time it takes for a field change to penetrate through an object to be measured and be detected on the other side, is very short and hence in practice difficult to measure reliably using current technology. The reason for this is that the time of penetration is so short that it may be easily disturbed by other delays in the measuring system. For example, a certain delay in the electronic components of the measuring device itself always occurs.